Digital optical switch

ABSTRACT

A digital optical switch with two input terminals and two output terminals to satisfy the extensive uses of optical switches. The conditions for adiabatic mode evolution can be flexibly adjusted depending on the planned use for said switch, the technological production thereof still being simple. In the digital optical switch according to the invention, the first incoming waveguide, with regard to the second incoming waveguide in the input section and the third outgoing waveguide with regard to the fourth outgoing waveguide in the output section are symmetrical with each other in cross-section, index of refraction and arrangement in relation to the direction in which the light spreads. Structural electrodes have a tapering effect on the wave guides and are adjusted thereto, and an electrode arranged in the output section is electrically driven.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a digital optical switch including a firstsection, called input section, having a first input waveguide and asecond input waveguide, these waveguides converging toward each otherto, and engaging each other at, the point of their coupling range, and asecond section, called output section, having a third output waveguideand a fourth output waveguide, these output waveguides diverging fromeach other from this point to a point outside of their coupling range,the waveguides of the second section connecting to those of the firstsection, and means for controlledly influencing the state of lightpropagation in accordance with the adiabatic mode evolution.

Optical switches are attractive components for the connection ofbroad-banded optical signals in optically transparent networks, for theprotection of control circuitry for by-passing a defective system orcable, and for spatial switches in transparent optical network nodes.Switching of a TV cable in a local area network (LAN), of broad-bandoptical ports of a computer or of optical signals in sensors andautomobiles constitute further applications in the fields oftelecommunications, micro system technology and automotive vehicles.These different applications require distinct switching parameters, suchas low cross-talk and low switching power, independence of polarizationand wavelength immunity.

2. Prior Art

The state of the art from which the invention is proceeding, may betaken from U.S. Pat. No. 4,775,207 or Appl. Phys. Lett. 51 (16), Oct.19, 1987, pp. 1230-1232, and from European Patent 0 457 406.

The first-mentioned publications relate to a digital optical switchprovided with waveguides based upon LiNbO₃ and disposed in an X-shapedarrangement, the width of the two converging waveguides at their inputsection being different. The output waveguides, commencing at theirpoint of engagement, are embraced by electrodes. The two inputwaveguides in the input section and, in the output section, the twooutput waveguides extending from the point of engagement, are disposedrelative to each other at an angle Θ where Θ<<Δβ/γ, Δβ being the averagedifference of the propagation constants of the two symmetric modes and γbeing the transverse wave constant in the vicinity of the waveguides. Inthis four-gated digital electro-optical switch, the switching operationis based upon the adiabatic mode evolution with reference to asymmetricwidth of the input waveguides. In this manner only a predetermined modecan controlledly propagate in the waveguides at an appropriate switchingof the output waveguides.

In European Patent 0,457,406, there is described a digital opticalswitch in which the input waveguides in the input section are ofasymmetric shape and in which the output waveguides in the outputsection are asymmetric or symmetric relative to each other and may beelectrically switched, the asymmetry in the shape of the waveguides inthe input and output sections being realized by a straight and a curvedwaveguide converging toward or diverging from each other. The purpose ofthe curved shape of the waveguides converging toward each other in theinput section and diverging from each other in the output section is toreduce the length of the component relative to the previously describeddigital optical switch (DOS).

In the arrangements of 2×2 DOS's thus far described by the prior art theasymmetry required for the principle of adiabatic mode evolution hasbeen realized, at least in part, in the input and/or output sectionthereof during the its fabrication. Adiabatic light propagation ispossible only at small changes of the waveguide parameters so that theoptical energy impinging upon the switch in a given basic mode, ismaintained in this mode, i.e., no mode transformation occurs. If,therefore, optical energy of a low mode order impinges upon the switch,the light will be conducted through the output waveguide of the higherrefractive index, resulting in a higher extinction ratio if the opticalenergy propagates essentially adiabatically in the switch.

Furthermore, digital optical switches are known which are constituted by1×2-Y-branches. Thus, there was a report about a digital thermo-optical1×2 switch made of polymer (see: Proc. 21st Eur. Conf. on Opt. Comm., pp1063-1065) in which the waveguides are buried, the output branchesinclude an angle of 0.12°, and heating electrodes are arrangedcompletely to cover both branches. Upon heating an output branch lightwill be conducted into the unheated branch. An extinction coefficientbetter than 20 dB at a switching power between 130 mW and 230 mW wasmeasured in the unheated branch; at about 180 mW the extinctioncoefficient reached a value of 27 dB.

In another report from ECOC'95—Brussels a 1×8 DOS is described for thefirst time which is constructed of three cascades of 1×2 switches (see:Proc. 21st Eur. Conf. on Opt. Comm., pp. 1059-1062. This solution, too,makes use of the thermo-optic effect in polymeric waveguides, which at alow switching power is capable of effecting a large change of therefractive index and, hence, a controlled conduction of the mode.

OBJECTS OF THE INVENTION

It is the task of the invention to provide a digital optical switchhaving two inputs and two outputs in which the conditions for anadiabatic mode evolution are flexibly adjustable, depending on itsapplication and which may be fabricated by simple technology.

SUMMARY OF THE INVENTION

In accordance with the invention the task is accomplished by a digitaloptical switch of the kind referred to above in which the first inputwaveguide and the second waveguide in the input section and the thirdoutput waveguide and the fourth output waveguide in the output sectionare respectively arranged identically to each other as regards theircross-section, refractive index and symmetrical in their arrangementrelative to the direction of propagation, and in which structuredelectrodes of spatially variable width are arranged adjacent to thewaveguides and in which an electrode arranged in the input section andan electrode arranged in the output section are adapted to beelectrically energized.

Because of the arrangement of two identical waveguides each in the inputand output sections and because of the electrodes arranged adjacent tothe waveguides and exerting a tapering effect thereupon, the solution inaccordance with the invention, for the realization of an adiabatic modeevolution, makes possible a separate adjustability of the parameters ineach waveguide branch. In this manner, an asymmetric waveguidetransition may be realized without any need for preadjustment by way ofthe waveguide geometry in a section of the DOS during its fabrication,since both sections, the input as well as the output section, inaccordance with the invention are structured as switching sections. TheDOS in accordance with the invention which is based upon adiabatic modeevolution displays a stepped switching behavior which results in adefined switching state being maintained as long as an applied switchingvoltage or a current is above a certain threshold. Because of theflexible adjustability of its operating mode by the electrodes itsprovides for large fabrication tolerances and reduced the need for aprecise setting of the switching voltage or of a precise currentcontrol. Moreover, such a DOS is immune from wavelengths.

Embodiments in accordance with the invention provide for

the first input waveguide and the second input waveguide convergingtoward each other in the input section in straight lines at an angle Θ,where Θ<<Δβ/γ, wherein Δβ connotes the average difference between thepropagation constants of the two symmetrical modes and γ connotes thetransverse wave constant in the vicinity of the waveguides vicinity, andthe third output waveguide and the fourth output waveguide divergingfrom each other in straight lines at the same angle Θ in the outputsection or

the first input waveguide and the second input waveguide convergingtoward each other arcuately in the input section and the third outputwaveguide and the fourth output waveguide diverging from each otherarcuately in the output section; in a special arrangement of theembodiments thus far mentioned the input waveguides in the input sectionbeing identical as regards cross-section and refractive index andsymmetric in their arrangement relative to the output waveguides in theoutput section, or

the first input waveguide and the second input waveguide convergingtoward each other in straight lines at an angle Θ, where Θ<<Δβ/γ and thethird output waveguide and the fourth waveguide diverging from eachother arcuately in the output section, or

the first input waveguide and the second input waveguide convergingtoward each other arcuately in the input section and the third outputwaveguide and the fourth output waveguide diverging from each other instraight lines at angle Θ.

Other embodiments provide for the electrodes arranged adjacent to thewaveguides to cover the waveguides in a tapered manner or to bestructured in a taper-like manner and to be positioned in the same layeras the waveguides.

In these embodiments the solution in accordance with the inventioncomply with the waveguide materials mentioned in a further embodiment tobe selected from the group: III-V-semiconductors, LiNbO₃, glass,Si—Ge-hybrid crystals, SiO₂, polymers, in order to realize the desiredeffect in any variation, i.e. the controlled adjustability of thedifference in the speed of propagation of light in both waveguides ofthe input section based upon the thermo-optical or electro-opticaleffect as a function of the waveguide material. The ability to selectfrom a wide spectrum of materials for fabricating the digital opticalswitch in accordance with the invention leads to an even wider field ofapplication.

Further embodiments of the invention relate to control variants of theelectrodes. Thus, the electrode arranged adjacent to an input waveguidein the input section and the electrode arranged in the output sectionadjacent to the output waveguide positioned mirror symmetrically to thisinput waveguide or adjacent to the output waveguides positionedpoint-symmetrically to this input waveguide are adapted to beelectrically energized.

These embodiments of the invention which relate to the symmetricalarrangement of the input waveguides in the input section and of theoutput waveguides in the output section as well as to the controlledadjustment of an asymmetry in the light propagation between thewaveguides in the input or output sections by energization of theelectrodes and thus to the change in light propagation in the waveguidesarranged adjacent to the energized electrodes, ensure a great manyvariations in which the DOS in accordance with the invention may berealized.

The area in which the electrode exerts a tapering effect upon thewaveguide ensures the adiabatic waveguide coupling in the input andoutput sections. In connection with a first input waveguide and a secondwaveguide which approach and engage each other in straight lines at anangle Θ where Θ<<Δβ/γ in an input section and a third output waveguideand a fourth output waveguide which diverge from each other from thepoint of engagement in straight lines at the same angle Θ in the outputsection, the condition for the adiabatic mode evolution may be adjustedby energizing an electrode in the input section such that a controlleddifference Δβ in the speed of propagation of the light is generated inthe two input waveguides of the input section. If the angle Θ satisfiesthe conditions for an adiabatic mode evolution and if the angle issufficiently small, either the symmetrical mode or the asymmetricalmode, depending on whether the corresponding input waveguide is heatedor unheated, will be generated in the central region in which the pointis located at which the waveguides in the input section meet and fromwhich the output waveguides in the output section diverge. Accordingly,the symmetrical mode may be conducted from the central region to thedesired output waveguide by precisely setting the heating power of theelectrodes arranged adjacent to the output waveguides. The symmetricalmode will always propagate in the unheated waveguide and at a sufficientheating power the asymmetric mode will propagate in the heatedwaveguide. Hence, all waveguides in the input and output sectionsrealized in accordance with the invention are monomodal. The conditionfor the adiabatic mode evolution may be set analogously, if thewaveguide in at least one of the sections, i.e. the input or outputsection, of the DOS in accordance with the invention is formedarcuately.

In a matrix consisting of a plurality of prior art digital opticalswitches arranged in a cascade, taper regions are required because ofthe differing dimensions of the input waveguide branches. These taperregions would increase the structural length of the matrix in anundesirable manner and would not permit an optimum input attenuation.These disadvantages do not occur in the embodiment of the invention asit relates to the mirror and point symmetrical configuration of theinput waveguides in the input section relative to the output waveguidesin the output section and its realization of a bidirectionally operableDOS, since the field distribution need only be adjusted to across-section which for all waveguides is the same. The DOS inaccordance with the invention is, therefore, ideally suited for use as amatrix building block. Furthermore, the DOS in accordance with theinvention represents a very compact arrangement compared to the1×2-Y-switch mentioned in the prior art which as a matrix made up offour such switches would be capable of functioning in the manner of theDOS in accordance with the invention.

In a preferred embodiment, the digital optical switch in accordance withthe invention is fabricated on a polymeric basis. The angle Θ enclosedby the two linearly converging or diverging waveguides is ≦0.1°. Theelectrodes are arranged in a buffer layer over the waveguides and coverthem in a tapering manner. In this preferred embodiment, the refractiveindex of a given waveguide may be adjusted by Δn>0.0015, by energizationof the electrodes. Because of well established fabrication technologythe use of polymeric waveguides makes many different structurespossible. Moreover, polymers have a large thermo-optic coefficient,i.e., changes in temperature result in large changes of the refractiveindex, coupled with low conductivity. It is possible by means of polymertechnology to integrate, by way of hybrid technology, a plurality ofoptical components on a single substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will hereafter be described in greaterdetail with reference to the drawings, in which:

FIG. 1 is a schematic presentation in top elevation of a multi-layerstructure of a digital optical switch on a polymeric basis;

FIG. 2 is a section along line A—A′ of the multi-layer structure shownin FIG. 1;

FIG. 3 the simulated beam propagation method (BPM) for the bar state ofthe digital optical switch shown in FIG. 1;

FIG. 4 Depicts the simulated cross state of the digital optical switchshown in FIG. 1;

FIG. 5 is a measurement trace of the optical efficiency of thewaveguides of the output section as a function of the electrical powerfor energizing the electrodes of a digital optical switch in accordancewith FIG. 1, in its bar state;

FIG. 6 is a measurement trace of the optical efficiency of thewaveguides of the output section as a function of the electrical powerfor energizing the electrodes of a digital optical switch in accordancewith FIG. 1, in its cross state; and

FIG. 7 is a measurement trace depicting the adjustability of theswitching mode.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The multi-layer structure of a polymer-based digital optical switchshown in FIG. 1 is provided in its input and output sections S1 and S2with symmetrical waveguides of identical cross-section and refractiveindex. Two input waveguides WG1 and WG2 with corresponding input gates 1and 2 are arranged at an angle Θ=0.08° in the input section S1.Waveguides WG1′ and WG2′ with corresponding output gates 1′ and 2′diverging from each other at the same angle Θ are shown in the outputsection S2. The waveguides in the input section S1 are arrangedsymmetrically with respect to the waveguides in the output section S2.The waveguides were fabricated from PMMA on a 3″ Si wafer by standardpolymer waveguide technology. There cross-section measures 7 μm×7 μm.The input and output waveguides are spaced 250 μm from each other. Theoverall length of the digital optical switch is 25 mm. Each of the fourwaveguides WG1, WG2, WG1′ and WG2′ is taperingly superposed by anelectrode E1, E2, E1′ and E2′. The propagation properties of light inthe digital optical switch in accordance with the invention arecharacterized by an adiabatic mode evolution. The conditions for theadiabatic mode evolution can be realized for each waveguide WG1, WG2,WG1′ and WG2′ by energizing the corresponding electrode E1, E2, E1′ andE2′. As a result of the invention, a digital optical switch having twoinputs and two outputs can finally be fabricated on a polymer basis.

In FIG. 2, the 2×2 polymeric digital optical switch described supra intop elevation is depicted in cross-section. A SiO_(x) buffer layer isarranged on a Si substrate which serves as a heat sink. For thereduction of losses and for protection against moisture, a 3 μm thickbuffer layer TB made of Teflon® is arranged on the waveguide layer W inwhich the waveguides WG1, WG2, WG1′ and WG2′ are disposed. As a finallayer, a corresponding heating electrode E1, E2, E1′ and E2′ is arrangedin superposition over each waveguide WG1, WG2, WG1′ and WG2′ such thatoverlaps the waveguide in a tapering manner.

The function of the DOS in accordance with the invention which is basedupon adiabatic mode evolution, may be described particularly well withreference to FIG. 3 and FIG. 4. the first partial image of which depictsthe input and output sections S1, and S2 and the waveguides WG1, WG2,WG1′ and WG2′ arranged in an X-shaped configuration. It also shows whichelectrode is being heated. Th ensuing partial images depict the possiblepropagation of a lightwave in the individual waveguides WG1, WG2, WG1′,and WG2′ in their bar and cross states.

If as shown in FIG. 3, electrodes E2 and E2′ are heated, i.e. if theyare energized by a power of P_(el-E2) and P_(el-E2′), the digitaloptical switch will operate in its bar state. As the light conductedinto the unheated waveguide WG1 reaches the central region, thesymmetric mode -as shown in the second partial image- is conductedthrough waveguide WG1′ which has a higher index of refraction comparedto waveguide WG2′. If light is conducted into the heated waveguide WG2the asymmetric mode will in the central region be conducted into thewaveguide WG2′ which has a lower index of refraction. Between theunheated waveguide WG1/WG1′ and the heated waveguide WG2/WG2′ thedifference in refractive index was found to be Δn=0.0025.

If, as may be seen in the first partial image of FIG. 4, electrodes E1and E2″ are heated, i.e., energized at a power of P_(el-E1) andE_(el-E2′). the DOS in accordance with the invention will operate in itscross state. In this case, too, the same difference in refractive indexas mention above, was detected. If light is conducted into heatedwaveguide WG1 the asymmetric mode is excited in the central region andthe light will be conducted through the heated waveguide WG2′ which hasthe lower index of refraction. If light is conducted through theunheated WG2 the symmetric mode is excited in the central region andlight will be conducted through the waveguide WG1′. It was found thatthe adiabatic mode can be maintained in the waveguides in the inputsection as well as in the waveguides in the output section provided thatat an angle of Θ≦0.1° between the waveguides in the input or outputsection the difference in refractive index is Δn>0.0015.

In order to characterize the polymeric digital optical switch inaccordance with the invention, the schematic structure and function ofwhich were explained in FIGS. 1 and 2, and FIGS. 3 and 4, respectively,light from a laser diode at γ=1.55 μm was coupled into the input gate 1and into the input gate 2, and the optical power P_(opt) was measured atthe output gates 1′ and 2′. Since it was found that the TE and TMpolarization values were polarization dependent by <±0.5 dB only, onlythe results for the TM polarization have been indicated.

FIG. 5 thus shows the measurement trace of the transfer characteristicat a wavelength of γ=1.55 μm as a function of the electrical powerP_(el-E2′), i.e., in this case it was electrode E2′ which was heated.The electrode E2 was energized at a constant power P_(el-E2), which inthis case was 65 mW (the energization is depicted in the insertedimage). This is necessary in order to realize the adiabatic modeevolution within waveguides WG1 and WG2 of the input section 1. In thisconfiguration the switch operates in its bar state. The value measuredfor cross-talk was measured as <−25 dB at an electric switching power ofP_(el-E2′)≧45 mW. It was found that this value does not change forswitching powers of P_(el-E2′), 100 mW.

The transfer characteristic for a polymeric digital optical switch inaccordance with the invention as a function of electric power P_(el-E2′)for the cross state is shown in FIG. 6. In the case, electrode E1 isenergized by a constant electric power P_(el-E1)=45 mW in order again tosatisfy the conditions of adiabatic mode evolution in waveguides WG1 andWG2 of the input section S1. If the variably adjustable switching powerassumes values of ≧45 mW, here, too, the measured cross-talk was <−25dB. The insignificant deviation of the powers set as a constant in bothdescribed switching states must be assumed to be the result ofinsignificant manufacturing differences in the electrodes andwaveguides. This does not, however, affect the principle of theinvention to structure each waveguide such that it is transparent to onedefined mode only.

The switching time of the thermo-optical polymer DOS in both switchingconfigurations is <1 ms.

The defined transfer characteristic in the same switch was alsoestablished at γ=1.3 μm and displays a similar digital switching action(not shown).

Looking at FIG. 7 which depicts the transfer characteristics of thepolymer DOS in accordance with the invention with the same electrodeconfiguration as shown in FIG. 5 but at different values for P_(el-E2),it will be seen that the switching action of such a DOS may be adjustedby energizing an electrode in the input section at a variable switchingpower and by a constant basic energization of an electrode in the inputsection. It can be seen that the “digitality” of the DOS may be adjustedas a function of the constant value P_(el-E2) for the basic energizationof electrode E2 in input section S1, such that within a narrow range ofP_(el-E2′) values a very good extinction ratio is ensured. If this valueis changed a “robust” digital switching action at a low extinction ratiowill present itself.

The described switching action permits tolerances in the technologicalfabrication process and eliminates the need for a precise current orvoltage control. Furthermore, the switching action of the presented DOSis stable against changes in ambient temperature.

What is claimed is:
 1. A digital optical switch, comprising: an inputsection including first and second input waveguides providing forsubstantially symmetrical mode propagation and converging in the inputsection in a point in their coupling region; an output section includingthird and fourth output waveguides in the output section providingsubstantially symmetrical mode propagation and respectively connected tothe first and second input waveguides and diverging from the point; aselectively energizable structured electrode overlapping to each of thefirst and second input waveguides and the third and fourth outputwaveguides in a tapering manner thereby to affect an adiabatic modeevolution in the waveguide.
 2. The switch of claim 1, wherein the inputwaveguides and the output waveguides are of identical in cross-section.3. The switch of claim 1, wherein the input waveguides and the outputwaveguides are of identical refractive index.
 4. The switch of claim 1,wherein the waveguides are symmetrical in their arrangement relative tolight propagating therein.
 5. The switch of claim 1, wherein the firstand second input waveguides converge toward each other along straightlines at an angle of Θ<<Δβ/γ between them wherein Δβ is the averagedifference of the propagation constants of the two symmetrical modes andγ is the transverse wave constant in the vicinity of the waveguide andwherein the third and fourth output waveguides divert from each otheralong straight lines at the same angle Θ between them.
 6. The switch ofclaim 1, wherein the first and second input waveguides converge towardeach other in arcuate paths and wherein the third and fourth outputwaveguides divert from each other in arcuate paths.
 7. The switch ofclaim 1, wherein the first and second input waveguides converge towardeach other along straight lines at an angle of Θ<<Δβ/γ between themwherein γβ is the average difference of the propagation constants of thetwo symmetrical modes and γ is the transverse wave constant in thevicinity of the waveguide and wherein the third and fourth outputwaveguides divert from each other along arcuate paths.
 8. The switch ofclaim 1, wherein the first and second input waveguides converge towardeach other in arcuate paths and wherein the third and fourth outputwaveguides divert from each other along straight lines at an angle ofΘ<<Δβ/γ between them wherein Δβ is the average difference of thepropagation constants of the two symmetrical modes and γ is thetransverse wave constant in the vicinity of the waveguide.
 9. The switchof claim 1, wherein the electrodes cover the waveguides in a taperingmanner.
 10. The switch of claim 1, wherein the electrodes are of taperedconfiguration and are disposed in the same layer as the electrodes. 11.The switch of claim 1, wherein at least one of the input section and theoutput section comprises a plurality of layers in superposition andwherein the waveguides and the electrodes are disposed in differentlayers.
 12. The switch of claim 1, wherein the waveguides are made froma material selected from the group consisting of III-V semiconductors,LiNbO₃, glass, Si-GE-hybrid crystals, Si₂, and polymer.
 13. The switchof claim 5, wherein the switch is made of a polymer and the angleΘ≦0.1°.
 14. The switch of claim 3, wherein the waveguides are disposedin a first layer of the input and output sections and the electrodes aresuperposed on the waveguides in a tapering configuration in a bufferlayer.
 15. The switch of claim 3 wherein selective energization of theelectrode causes a change of Δn>0.0015 in the refractive index of theadjacent waveguide.